The invention is directed to a process for the hydrophobization of pyrogenically produced silica, preferably in direct connection with the process of the production of pyrogenic silica from a silicon halide compound.
It is known according to German Pat. No. 1163784 to hydrophobize pyrogenically produced silica with dimethyldichlorosilane in a fluidized bed. The entire disclosure of the German patent is hereby incorporated by reference and relied upon.
The individual components according to the process scheme of this publication are mixed with each other in such manner that steam in admixture with nitrogen is mixed with the pryogenically produced silica. Subsequently this wet silica is supplied with a gaseous mixture consisting of dimethyldichlorosilane and nitrogen. The true hydrophobization reaction is carried out subsequently in a fluidized bed reactor.
This procedure has the disadvantage that because of the moisture content of the pyrogenically produced silica the pipelines of the hydrophobization apparatus are stopped up.
Furthermore, high loss of yields and undesired sideproducts occur through the partial reaction of the organohalosilane with the steam on the pryogenically produced silica before the hydrophobization reaction can be carried out under controlled conditions in the fluidized bed reactor.
A further disadvantage of the known process according to German Pat. No. 1163784 results from the specific composition of the waste gas of the hydrophobization reaction. It consists of hydrophobized pyrogenically produced silica, excess organohalosilane, hydrogen chloride, if for example, there is reacted dimethyldichlorosilane as the organohalosilane, nitrogen and steam.
Hydrophobic silica is led into the hydrogen chloride-absorption apparatus by means of the waste gas return described in the drawing of German Pat. No. 1163784. Through this there occurs an undesired formation of foam.
The waste gas lines are clogged up through polymerization reactions of the organohalosilane. Through this there occur disturbances in the plant such as pressure loss and fires.
In the experiment of working up this waste gas by means of water washing in a cooling tower there occurs a severe formation of foam. This foam is recirculated in the cooling tower whereby through the hydrophobic pyrogenically produced silica there is formed "dry water". It is not possible to filter off this foam because the filter is clogged up too quickly.